The present invention relates to a parallel proportioning valve for use in the fluid circuit of a dual braking system for a motor vehicle.
U.S. Pat. No. 3,423,936 owned by the Kelsey-Hayes Company discloses a proportioning valve comprising a chamber having an inlet connected to a pressure source and an outlet connected to a rear brake cylinder which is supplied with a reduced pressure and having a pressure responsive plunger accommodated within the chamber for opening and closing a fluid passage between the inlet and outlet and a spring for urging the plunger in the direction to open the fluid passage.
When the proportioning valve of this type is used in a fluid circuit of a dual braking system, a pair of such valves are disposed therein.
In order to save material and manufacturing costs, a valve device incorporating a pair of proportioning valve assemblies in a single casing is provided. However, the valve device is constructed so that each of the valve assemblies has its own spring for urging the pressure responsive plunger. This arrangement makes it difficult to compensate for normal differences in cut in points of the pressure reducing action of each valve assembly.
In addition, when one of the two systems becomes inoperative due to, for example, fluid leakage, it becomes preferable to compensate for the inoperative value by increasing the effective pressure being supplied to the rear brake cylinder by the other valve assembly which is in operation thus appropriately distributing the pressurization of the front and rear brakes.
However, since each of the valve assemblies has its own spring forcing each of the plungers, when one of the two braking systems becomes inoperative, the valve assembly in the other system continues to operate so that the reducing action will occur at a predetermined normal point without adjusting the cut-in point to a higher level of pressure.